An optical card, used as various data memory cards, such as cash cards, credit cards or identification certificate cards, is of such a construction that a recording layer on which are recorded optically readable data is formed with the interposition of a protective film and a protective layer is laminated on the recording layer with the interposition of a protective layer. On the surface of the protection film, there is printed either information on the method of using the cards or decorative patterns.
The principle of recording and reproducing data in the optical card is to distinguish the difference in the transmissivity or reflectivity of the recording layer and to convert the difference into electrical signals. As the recording layer, thin films of metal or alloy or thin films of organic materials, such as dyestuffs, capable of optically recording or reproducing data, are employed. The recording layer need be formed as an optically homogeneous medium. In general, the recording layer formed by a thin film of the metals or alloys is formed on a base material by vacuum film forming techniques, such as evaporation or sputtering, whereas the recording layer formed by a thin film of the organic material is formed by coating the organic dyestuff on the base material by a spin coater or a roll coater.
The above mentioned protective layer for mechanical protection of the recording layer and maintenance of environmental properties, such as resistance to high temperature, high humidity or chemical pollution, is generally formed of an UV curing resin.
The UV curing resin, now used extensively, is superior in general properties as a protective layer, however, it has a higher Young modulus (modulus of elasticity) such that it is deformed markedly against mechanical stresses such as compression, tension or bending and unable to assure sufficient protection of the recording layer.
In general, the UV curing resin has a high cross-linking density and a higher thermal conductivity, as thermal properties, which are not desirable as recording properties, because they affect recording sensitivity. That is, when recording with a semiconductor laser element, for example, for forming a predetermined recording pit, the laser power may fall short depending on the type of the protective layer. Above all, in optical recording, with an optical card in need of a larger pit and low speed recording conditions, it is critical to elevate the recording sensitivity, such that a demand is raised for the resin of the protective layer having these thermal properties.
On the other hand, for producing the above mentioned optical card, necessary data are printed in advance on a roll-shaped protective film, such as by silk screen printing or offset printing, the printed protective film is laminated on a recording layer provided on the base material, and the optical card is punched from the laminated base material to a desired shape.
With the conventional method, in which the protective film having the necessary data printed thereon is laminated on the recording layer formed on the base material and the optical card is punched from it to a desired shape, it is necessary to realize high precision positioning of the protective film with the base material having the recording layer formed thereon. For example, it is necessary to provide positioning markers in the base material and the protective film and to perform a positioning operation on the basis of these markers. When various data are provided by multi-color printing on the protective film, a number of printing steps equal to the number of colors is necessitated, and the roll of the protective film must be rewound and set on a printer for each printing step, while a high precision is required for positioning in the printing process. For printing on the protective film by the silk printing or offset printing methods, a professional grade of a printing equipment is necessitated. Since a high precision is necessitated in positioning with the conventional manufacture method, the yield of production of optical cards is low and production costs are elevated, so that the conventional method is not suited to the multiple species, small quantity production of optical cards.
On the other hand, an optical card is-usually formed in a rectangular shape and, if such card is prepared one by one using a rectangular base material, it is difficult to form an optically homogeneous recording layer by vacuum film forming or spin coating during the production process, such that it is difficult with the conventional manufacture method to stably produce high quality products. Above all, in case of spin coating, paints proceed to the reverse sides at the corners of the rectangular base material to lower the production yield of the rectangular base material and to raise the production costs.
It is therefore an object of the present invention to provide an optical recording medium, such as an optical card or an optical disk, which is improved in recording sensitivity or reliability. It is also an object of the present invention to enable a high quality optical recording medium to be produced stably so that the production yield is elevated to lower the production costs and the multiple species small quantity production is facilitated.